The overall c-section rate rose rapidly in the United States in the 1960s, 1970s, and most of the 1980s. The rate leveled off in the late 1980s, and began a slow decline, which continues according to the most recent data available. It stood at 20.8 percent in 1995, a rate still regarded by many authorities as too high. The high c-section rate of the 1980s attracted widespread concern, both on medical and on economic grounds. What was responsible for the high rate? To what extent did economic factors play a role? The leveling off and decline of the c- section rate has also raised important questions. Exactly what is the nature of the change? What is its explanation? The proposed research seeks to answer such questions, within the context of a carefully designed determinants study using an unusually rich data set. Our specific aims are to construct and develop an appropriate data set for New York State for 1986 and the most recent available year, probably 1996, and for each year to perform a two-stage regression study of the determinants of the probability of cesarean delivery; and then to use the results for a simulation exercise, to determine the causes of changes in cesarean sections between 1986 and 1996, distinguishing between changes in behavior (system parameters) and changes in variables (system independent variables). The two stages consist of determinants of use of the labels fetal distress and dystocia as indications for cesarean section; and (using predicted values of these indications among the regressors), determinants of the probability of cesarean delivery. There is evidence suggesting use of these labels is endogenous. In the simulation, the 1996 variables are used with the 1986 parameters, to determine how many cesarean sections would have occurred in 1996 if 1986 behavior had been maintained, and what the cesarean section rate would have been. The result can be compared with the actual 1996 values. The difference between the hypothetical and the actual number of cesarean sections is interpreted as the additional (positive or negative) 1996 cesarean sections attributable to 1986-1996 behavior change. The same is true of the c-section rate. The research contributes to our understanding of cesarean delivery decisions; and to our understanding of the influence of non-clinical factors on clinical decisions, including choice of diagnosis as well as of treatment. Among other things, this makes it easier to understand the contributions that policy has made and can make in this important area.